In recent years, camera modules for taking photos have begun to be incorporated in mobile terminals such as mobile phones and lap-top computers. Downsizing the camera modules is a prerequisite for enhancing the portability of these apparatuses. The camera module operates with an image pickup device such as a CCD (Charged Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor). Recently, a pixel having the size of approximately a few micrometers has become commercially feasible, and an image pickup device with high resolution and a compact size can now be commercialized. This is accelerating the demand for downsizing of image pick-up lens systems so that they are able to be suitably used with miniaturized image pick-up devices. It is also increasing expectations of cost reductions in image pick-up lens systems, commensurate with the lower costs enjoyed by modern image pickup devices. All in all, an image pick-up lens system needs to satisfy the oft-conflicting requirements of compactness, low cost, and excellent optical performance.
Compactness means in particular that a length from a lens edge of the lens system to an image pick-up surface should be as short as possible.
Low cost means in particular that the lens system should include as few lenses as possible; and that the lenses should be able to be formed from a resin or a plastic and be easily assembled.
Excellent optical performance can be classified into the following four main requirements:
First, a high brightness requirement, which means that the lens system should have a small F number (FNo.) Generally, the FNo. should be 2.8 or less.
Second, a wide angle requirement, which means that half of the field of view of the lens system should be 30° or more.
Third, a uniform illumination on the image surface requirement, which means that the lens system has few eclipses and/or narrows down an angle of incidence onto an image pick-up device.
Fourth, a high resolution requirement, which means that the lens system should appropriately correct fundamental aberrations such as spherical aberration, coma aberration, curvature of field, astigmatism, distortion, and chromatic aberration.
In a lens system which satisfies the low cost requirement, a single lens made from a resin or a plastic is desired. Typical such lens systems can be found in U.S. Pat. No. 6,297,915B1 and EP Pat. No. 1271215A2. However, even if the lens has two aspheric surfaces, it is difficult to achieve excellent optical performance, especially if a wide angle such as 70° is desired. Thus, the single lens system can generally only be used in a low-resolution image pick-up device such as a CMOS. In addition, a thick lens is generally used for correcting aberrations. Thus, a ratio of a total length of the lens system to a focal length of the lens (L/f) is about 2. In other words, it is difficult to make the lens system compact.
In a lens system which satisfies the excellent optical performance requirement, three lenses are desired. A typical such lens system can be found in U.S. Pat. No. 5,940,219. However, the ratio of a total length of the lens system to a total focal length of the three lenses (L/f) is about 2. It is difficult to make the lens system compact. In addition, the plurality of lenses increases costs.
In order to satisfy all the requirements of compactness, low cost and excellent optical performance, it is commonly believed that a two-lens system is desirable.
A well-known two-lens system is the retro-focus type lens system. A typical such lens system can be found in U.S. Pat. No. 6,449,105B1. The lens system comprises, from an object side to an image side, a first meniscus lens having negative refracting power and a convex surface on the object side, a stop, and a second meniscus lens having positive refracting power and a convex surface on the image side. The lens system helps correct wide angle aberrations. However, a shutter is positioned between the second lens and the image side, which adds to the distance between the second lens and the image side. Thus, the compactness of the lens system is limited.
U.S. Patent Publication No. 2004/0036983 discloses an image pick-up lens which overcomes the above described problems. As represented in FIG. 30 hereof, the image pick-up lens comprises, from an object side to an image side: an aperture stop 1; a biconvex positive lens 2; and a meniscus lens 3 having a concave surface on the object side. When each of the lenses 2, 3 has at least one aspheric surface, the image pick-up lens satisfies the following conditions: 0.3<f1/f<0.9 and T/f<2.4. In these expressions, “f” is an overall focal length of the lens system, “f1” is a focal length of the positive lens 2, and “T” is a length from the aperture stop 1 to an image pick-up surface 5.
However, the ratio of the total length of the lens system to the total focal length of the lenses 2, 3 (L/f) is generally about 2. The smallest ratio obtainable is 1.7, which still constitutes a limitation on the compactness of the lens system. In addition, it is difficult to correct lateral chromatic aberration effectively, and thus the optical performance of the lens system is limited.
Therefore, a low-cost image pick-up lens system which can properly correct aberrations and has a compact configuration is desired.